Key-joint forming divider strips of the general type to which the present invention relates are used worldwide in the formation of concrete slabs for use in concrete airfield runways, roadways, building floors and the like and in general are in the form of elongated sheet metal pieces, the lower regions of which are formed with coextensive laterally offset portions in order to establish the desired key-joint in the adjacent poured concrete slabs and the upper regions of which folded over or bent portions in order to form upstanding screeds which have horizonally linearly straight upper edges and are adapted to support screed boards in connection with concrete levelling operations. In between such lower and upper regions of the strips, there are usually provided recesses, sockets or downward facing channels for reception of the upper ends of the stakes which are used in connection with the strips. The nature of such stake-receiving recesses, sockets or channels varies widely in connection with both the stake-receiving and screed-forming facilities of previously designed and used key-joint forming divider strips.
In the past, many such key-joints have been proposed and have been generally accepted in the trade, however, each of them have inherent problems which the present invention addresses.
One of the problems experienced with these products has been the possibility of the form not gripping the stake regidly enough to prevent the form from slipping and floating off before or during the pouring of the concrete.
Also, installing these forms are labor intensive and many require special tools for crimping the lower edge of a downturned flange into the aperture for locking the form onto the stake such as U.S. Pat. No. 3,288,042 or the device of U.S. Pat. No. 3,401,612 which requires a notch in each of the opposite sides of the stake so that a tool must be used to deform the lower edge of the downturned flange until it interlocks with the notch and makes a mechanical connection between the form and the stake.
Many of the prior art patents such as U.S. Pat. No. 3,784,313 depend on the height of the stake for vertical adjustability which requires ideal working conditions, close tolerances and provides a weak locking action between the form and its supporting stakes.
In other cases such as U.S. Pat. No. 1,345,179 the concrete form is hung from a bracket member on each stake, leaving the stake protruding above the top edge of the concrete forms so that a screed cannot be used as in the present invention, to move across the top edge of the key-joint that serves as a leveling guide to smooth out the concrete surface.
U.S. Pat. No. 4,455,104 attempts to address these problems by providing a stake and form arrangement which interlocks, however only a limited amount of vertical adjustability is possible, thus this device again depends on the height of the stake.